1. Field of the Invention
The present invention relates to the technology field of inverter controlling methods, and more particularly to a load impedance estimation and repetitive control method capable of allowing inductance variation for inverter.
2. Description of the Prior Art
With high developments of sciences and technologies, the requirements on electric power and power quality made by high-tech equipment grow with passing days. In order to avoid the execution of normal works from influence, it is extremely important to ensure the continuity and stability of the electric power supplied to the semiconductor manufacture processing equipment or the network server computers. For this reason, an uninterruptible power system (UPS) is provided for supplying back-up electricity to the electrical equipment when the electric grid is at an abnormal situation.
For enhancing the efficiency of the UPS, researchers propose a variety of control method for the controller of the UPS, such as dead-beat control, sliding mode control, pole-placement algorithm, and repetitive control. In above-mentioned methods, although the dead-beat control is able to increase the response speed of the inverter, the dead-beat control cannot effectively eliminate the harmonic distortion from the output current of the inverter when a non-linear load is connected to the inverter. Besides, in spite of the sliding mode control can enhance the dynamic response of the inverter by way of feed-forward compensation, it is difficult to determine a suitable sliding surface for the sliding mode control. Moreover, both the pole-placement algorithm and the repetitive control have the same shortcoming of complex mathematical derivation.
In 2002, Fred C. Lee proposed a 3D SVPWM (Three Dimensional Space Vector Pulse Width Modulation) method for controlling the inverter; therefore, a voltage-type three-phase four-wire inverter based on SVPWM was carried out through defining the vector selections for each of time sequence intervals. To further understand the conventional controllers, the state equations are derived based on the constant three-phase inductance, so that some compensation must be used for overcoming the inadequate inductances, in which the inductances decrease with the increase of the output current; eventually, the whole inverter system is put at risk of system divergence. Moreover, the way of Park Transformation doing in SVPWM is to transform the RST coordinates of three-phase three-wire system to DQ coordinates, or transform the RSTN coordinates of three-phase four-wire system to αβγ coordinates.
However, because the Park Transformation is completed by taking the voltage signal and current signal of three phases as fundamental sinusoidal functions, harmonic compensator or predictor must be added in to the system for compensating the inadequate transformations, from which the voltage signal or current signal of three phases include harmonic components; eventually, the design of the controller for the inverter system is getting complicated. Besides, since the feedback compensation in the conventional inverter system is designed corresponding to a specific load, the previously designed feedback compensation would become unsuitable, at which a new load differing from the specific load is connected to the inverter.
Accordingly, in view of the conventional inverter controlling methods including obvious drawbacks and shortcomings, the inventor of the proposed approach has made great efforts to make inventive research thereon and eventually provided a load impedance estimation and repetitive control method capable of allowing inductance variation for inverter.